Isocyanates are produced in large volumes and serve mainly as starting materials for production of polyurethanes. Since the standard monomeric diisocyanates have a relatively low molar mass and generally correspondingly higher vapor pressure, polyisocyanates prepared therefrom are used for reasons of occupational health, particularly in paint production. These polyisocyanates are, for example, uretdiones, isocyanurates, iminooxadiazinediones, biurets, urethanes, allophanates or ureas, which are prepared from the monomeric diisocyanates by di- and trimerization, generally in the presence of catalysts. For this purpose, however, particularly high demands are placed on the purity of the monomers, since the secondary components typically present therein reduce the activity of the catalysts, significantly in some cases. It is therefore necessary to employ higher catalyst concentrations or longer reaction times, which distinctly worsens the quality of the resulting polyisocyanates, for example with regard to color and storage stability.
It is therefore desirable that a minimum level of secondary components have formed at the early monomeric diisocyanate production stage, in order then to limit the cost and inconvenience associated with the removal or minimization thereof, for example by fractional distillation.
In the case of pentane 1,5-diisocyanate, particularly the chlorinated secondary components 5-chloropentyl isocyanate (CPI), N-carbamoylpiperidine (“C6-Im”) and the two isomeric N-carbamoyltetrahydropyridines (“C6-Az”) are formed. While the formation of CPI reduces the yield and CPI is troublesome as a chain terminator in further processing because of its monofunctionality, particularly the C6-Im and C6-Az components, which contribute to what is called the HC (hydrolyzable chlorine) value, impair the catalysis in the further processing of PDI to give polyisocyanates, and so the HC value of the monomers used for preparation of the polyisocyanates should always be <100 and preferably <50 ppm.

In the PDI which is used for preparation of polyisocyanates, the concentration of CPI should be <0.3% and the sum total of the concentrations of C6-Im and C6-Az should not exceed 400 ppm, preferably 200 ppm. Since the removal of C6-Im and C6-Az from the PDI, for example by distillation, is very difficult and inconvenient, the concentration thereof in the crude materials as well should not be significantly higher.
The preparation of pentane 1,5-diisocyanate (PDI) from pentane-1,5-diamine(PDA) is known per se and can be effected in a phosgene-free manner (T. Lesiak, K. Seyda, Journal für Praktische Chemie (Leipzig), 1979, 321 (1), 161-163) or by reaction with phosgene (for example W. Siefken, Justus Liebigs Ann. Chem. 562, 1949, p. 75 ff., (p. 122) or DE 2 625 075 A1).
In the case of the above-cited phosgene-free preparation, PDA is first reacted with formic acid to give the formamide and then oxidized with halogen in the presence of tertiary amines to give PDI. A disadvantage of this process is that it is a complex two-stage process, wherein by-products are formed to a considerable extent. The resultant yield losses and the high purification complexity required reduce the economic viability of this process. DE 2 625 075 A1 claims a process for preparing carbamoyl chlorides and isocyanates, characterized in that salts of primary amines are reacted with phosgene in solid form in the presence of a liquid at elevated temperature in a rotary oven, a paddle drier or a fluidized bed reactor. A disadvantage of this process is that this too is a multistage process in which, in the first stage, an amine salt is first prepared in a solvent which then has to be removed again prior to the reaction with phosgene, for example by filtration or centrifugation and subsequent drying. This is time-consuming and costly and reduces the economic viability of this process.
DE 1 900 514 A1 describes the two-stage preparation of PDI from caprolactam by conversion to the hydroxamic acids and the subsequent phosgenation thereof. The yield reported in this document for the conversion of caprolactam to PDI is only about 32%.
WO 2008/015134 A1 claims a process for preparing PDI in which biobased lysine is converted to PDA, which is subsequently converted to PDI. The conversion of PDA to PDI can be effected in a phosgene-free manner or in the presence of phosgene, and the latter variant can be effected in the liquid phase or in the gas phase. Any impurities present in the PDI and measures for the avoidance or minimization thereof are not mentioned.
EP 2 684 867 A1 claims pentane 1,5-diisocyanate (PDI) having a content of 5-400 ppm of compounds (1) and (2) by cold-hot phosgenation of biobased pentane-1,5-diamine (PDA) or a salt thereof, a process for preparation thereof and polyisocyanates prepared thereby.

There is a description therein of the phosgenation of pentane-1,5-diamine salts, for example hydrochlorides, in inert solvents, for example o-dichlorobenzene, wherein the crude PDI thus obtained is conditioned to lower the content of compounds (1) and (2) prior to distillation by heating in the presence of an inert gas, for example nitrogen, and optionally a phosphorus compound, for example tris(tridecyl) phosphite, to 180-245° C. Nothing is said about the presence of the secondary component CPI or the removal thereof. This process also comprises several steps and requires long reaction times, which has an unfavorable effect on its economic viability.
There is therefore still a great need for a simple and inexpensive process for preparing PDI with sufficiently low contents of CPI, C6-Im and the two isomeric C6-Az species, which avoids the disadvantages of the prior art processes.